With rising concern about the number and impact of illegal aliens in the United States, criminal illegal aliens have become a subject of particular focus. These individuals have not only entered or resided in the United States without the knowledge or permission of the U.S. Government, but, while here, they have also violated the laws of the Nation, its States, and municipalities. At State and local levels, the costs of arresting, prosecuting, sentencing, and supervising criminal illegal aliens have become a major issue. The Federal Government has taken steps to reimburse States for some of the costs associated with criminal illegal aliens. Section 510 of the Immigration Reform and Control Act of 1986 (IRCA) authorized the Attorney General to reimburse the States for the criminal justice costs attributable to undocumented persons. No appropriations for illegal aliens were made until 1994, when the Violent Crime Control and Law Enforcement Act authorized $1.8 billion over 6 years to reimburse States for criminal justice costs associated with illegal aliens. The State Criminal Alien Assistance Program (SCAAP) was established to allocate and distribute these monies. This research project examined the characteristics of illegal aliens at the Federal, State, and local levels. A goal of this project was to be as nationally representative as possible, given the limits of existing data sets. The Federal-level analysis was based on two data sets, from the Pretrial Services Act Information System (PSAIS) and the U.S. Sentencing Commission (USSC), which were representative of individuals involved in the Federal criminal justice system. The State-level analysis was based on data collected for the SCAAP on the seven States with the most illegal aliens: Arizona, California, Florida, Illinois, New Jersey, New York, and Texas. Key findings of the study include information in the following areas: (1) the number of illegal aliens in prison and elsewhere in the criminal justice system; (2) between 1991 and 1995, how and why has the number of illegal aliens entering the criminal justice system changed; (3) what types of offenses have illegal aliens been convicted of and how do these types of offenses compare to the general population? (4) what types of illegal aliens are in the criminal justice system, and what are their characteristics? and, (5) what are the socioeconomic and demographic characteristics of illegal aliens in prison or in other parts of the criminal justice system, and how do they compare with others in the criminal justice system? The final section in this article discusses the problem of identifying illegal aliens, why the number of illegal aliens in the Federal criminal justice system has increased, and implications for developing strategies for reducing crimes committed by illegal aliens.